In order to improve their processability, cold-rolled steel sheets are heat-treated with a continuous annealing furnace having, in sequence, a heating zone, a soaking zone, first and second cooling zones, and, possibly an overageing zone, and through which the strip runs continuously.
The heating zone of the continuous annealing furnace can include a direct-fired annealing furnace or a radiant tube annealing furnace. These two types of annealing furnaces can be used alone or in combination to heat the strip up to its recrystallization temperature. However with a direct-fired annealing furnace or with a radiant tube annealing furnace, it is difficult to control the temperature of the strip and to insure a good homogeneity of its temperature all along the surface of the strip. Moreover, the use of these furnaces to heat the sheet can lead to the formation of oxides on the surfaces of the sheet, which must then be eliminated by additional pickling and/or shot blasting steps.
In order to solve these problems document FR-A-2 524 004 has disclosed a process for annealing a running steel strip in which said strip, instead of running through a furnace, runs through a molten glass bath kept at 950° C. or higher. The strip is then taken out of the molten glass bath with a coagulated glass coating formed on the surfaces of the strip, and then the strip is cooled down to a temperature lower than 400° C., preferably 300° C. or less, in order to destroy and peel off the glass coatings. The bath has a viscosity not exceeding 20 Pa·s as measured at 950° C. The cooling step is performed, for example, by projecting a gas, preferably an inert gas, or liquid water, onto the strip surface.
This method allows annealing the strip without surface oxidation, but it requires keeping the glass bath at a high temperature, and so requires a significant amount of energy. Moreover, at these high temperatures the molten glass composing the bath evaporates. The vapours are noxious, and they must be collected. Also, the bath must be regularly refilled, not only because the glass deposition onto the strip must be compensated, but also because the evaporation leads to a supplemental consumption.
This method also includes the formation of a glass coating on the surfaces of the strip which, as said before, implies additional steps of cooling of the strip at a temperature lower than 400° C., and of elimination of the glass coatings. These steps slow down the production of steel strips, and the cooling at a temperature lower than 400° C. implies that the running strip must be reheated if a galvanization is required in a following step of the treatment.